Correlation between the Shortened Period of Cell Pairing during Genomic Exclusion and the Block in Posttransfer Nuclear Development in Tetrahymena thermophia.. (conjugation/cell contact/star strains/nuclear transfer/Tetrahymena)

Developmental progression of Tetrahymena through the cell cycle and conjugation

The ciliate Tetrahymena thermophila can be said to undergo a variety of developmental programs. During vegetative growth, cells coordinate a variety of cell-cycle operations including macronuclear DNA synthesis and a-mitotic fission, micronuclear DNA synthesis and mitosis, cytokinesis and an elaborate program of cortical morphogenesis that replicates the cortical organelles. When starved, cells undergo oral replacement, transformation into fast-swimming dispersal forms or, when encountering cells of a complementary mating type, conjugation. Conjugation involves a 12 hour program of meiosis, mitosis, nuclear exchange and karyogamy, and two postzygotic divisions of the fertilization nucleus. This chapter reviews experimental data exploring the developmental dependencies associated with both vegetative and conjugal development.

Inhibition of oral morphogenesis during conjugation of Tetrahymena thermophila and its resumption after cell separation

During the early stages of the conjugation of Tetrahymena thermophila, the oral apparatus undergoes partial regression: the deep fiber bundle is resorbed and oral membranelies are partially deciliated. However, further progress of the oral replacement is arrested until cell separation. Only then proliferation of basal bodies for a new oral primordium is unblocked and concomitantly the old oral structures are resorbed. This step is followed by the assembly of new oral membranelies and resorption of the old ones. Thus, the cell separation itself may be the source of an important signal for the cortical morphogenesis. To test this hypothesis we have used micronuclearly defective "✴" cells which separate after conjugation about 2.5 h earlier than normal cells and do not undergo postzygotic nuclear divisions. In the "✴" crosses (the crosses of the "✴" × wt cells) the oral primordium appeared before the cell separation (at the stage of hemikarya) following partial regression of the old oral structures, but only in those pairs in which the separation was already advanced in the separation. Consequently onset of conjugational morphogenesis in the "✴" pairs was correlated with the time of the cell separation, in spite of the differences in the course of nuclear events in the two kinds of pairs.

Inactivation of a macronuclear intra-S-phase checkpoint in Tetrahymena thermophila with caffeine affects the integrity of the micronuclear genome

Aphidicolin (APH), an inhibitor of DNA polymerase α, arrested cell divisions in Tetrahymena thermophila. Surprisingly, low concentrations of APH induced an increase of macronuclear DNA content and cell size in non-dividing cells. In spite of the cell size increase, most proliferation of basal bodies, ciliogenesis and development of new oral primordia were prevented by the APH treatment. The division arrest induced by APH was partly overridden by caffeine (CAF) treatment, which caused the fragmentation ("pulverization") of the chromosomes in G2 micronuclei. Somatic progeny of dividers with pulverized micronuclei (APH+CAF strains) contained aneuploid and amicronucleate cells. The amicronucleate cells, after losing their oral structures and most of their cilia, and undergoing progressive disorganization of cortical structures, assumed an irregular shape ("crinkled") and were nonviable. "Crinkled" cells were not formed after APH + CAF treatment of the amicronuclear BI3840 strain, which contains some mic-specific sequences in its macronucleus. Most of the APH +CAF strains had a typical "*"- like conjugation phenotype: they did not produce pronuclei, but received them unilaterally from their mates and retained old macronuclei. However, 4 among 100 APH+CAF clones induced arrest at meiotic metaphase I in their wt mates. It is likely that the origin of such clones was enhanced by chromosome pulverization.

Isolation of a Tetrahymena thermophila Strain which Induced Metaphase I Meiotic Arrest: New Pathway of Abortive Conjugation

A hypodiploid strain of Tetrahymena thermophila has been obtained that shows arrest at the stage of condensed nuclei, corresponding to metaphase I of normal conjugants and induced arrest at meiotic metaphase I (i.e. at the stage of condensed, bivalent chromosomes) in its wt partner mate. The metaphase I arrested conjugants retained their old macronuclei and most of them underwent cell fusion, instead of separation of exconjugants. The doublets were viable and cortically integrated. When the arrest inducing strain was crossed to the haploid tester strain, the haploid micronuclei were arrested in the meiotic metaphase I as the diploid ones had been; the monovalent, chromosomes were condensed, the arms of sister chromatids were not separated, and they were not segregated. Separation of the arms of sister chromatids and disjunction of bivalent chromosomes were not prerequisite for the formation of microtubular spindles in those cells that were arrested in meiotic metaphase I. After re-feeding, the doublet cells resumed cell divisions, segregating two macronuclei and micronuclei at random. One macronucleus was derived from the arrest inducing strain and the other from the tester strain. Heterokaryon strains with macronuclei derived from the parental arrest inducing strain and with the micronucleus derived from the parental wt tester strain were obtained. Surprisingly, these heterokaryons did not induce meiotic arrest. Thus, the arrest in the melotic metaphase I was induced by the micronucleus and not by the macronucleus of the arrest inducing strain.

Development in electrofused conjugants of tetrahymena thermophila

Electric shock can create parabiotic fusions of living Tetrahymena cells. In this study, cells were mated and successful pairs were electrofused with either vegetatively growing cells or other mating pairs. In particular, we electrofused pairs from normal [diploid x diploid] matings with vegetatively dividing cells in G- or M-phase of the cell cycle. We also fused [diploid x diploid] conjugants with mating pairs involving an aneuploid partner [diploid x "star"], which typically undergo an abortive conjugal pathway termed genomic exclusion. Using such parabiotic fusions we identified and characterized two developmentally critical landmarks: 1) the "abort" signal, which is initiated in pairs with nuclear defects (this first becomes evident soon after the completion of Meiosis I or the beginning of Meiosis II); and 2) the "terminal commitment point", a developmental stage in normal [diploid x diploid] pairs after which conjugation no longer responds to a parabiotically transmitted abort signal (this correlates with the onset of the second postzygotic nuclear division). Finally we demonstrate that a conjugal-arrest-activity varies with the vegetative cell cycle, reaching its highest level of activity during M-phase and dropping just after cytokinesis.

Abortive conjugation induced by UV-B irradiation at meiotic prophase in Tetrahymena thermophila

Conjugating Tetrahymena were irradiated by ultraviolet-B (UV-B) at various stages of conjugation. When the conjugants were exposed to the UV-B at late meiotic prophase (the stage from pachytene to diplotene), abortive conjugation was induced a high frequencies. After completing meiosis, a significant number of the conjugants showed marked anomalies, i.e., failure of nuclear selection after meiosis, and abortion of the subsequent conjugation process such as a postmeiotic division to form gametic nuclei, nuclear exchange, synkaryon formation, and postzygotic development. The conjugating pairs retained the parental macronucleus and separated earlier as compared with a control. The resultant exconjugants degenerated meiotic products and became amicronucleates. These observations strongly suggest the presence of a UV-sensitive molecule that is expressed specifically at the meiotic prophase and that directs the subsequent development after meiosis.

Macronuclear development in conjugants of Tetrahymena thermophila, which were artificially separated at meiotic prophase

Conjugant pairs of Tetrahymena thermophila were mechanically separated by vigorous pipetting at the early stages of meiotic prophase. The complete sequence of conjugational nuclear events including the appearance of pronuclei, development of the new macronuclei (postzygotic development), and resorption of the old macronuclei was observed in the separated cells, without pronuclear exchange. The pronuclei in the separated cells were recognised by the presence of components of the extranuclear cytoskeleton, which were labelled with anti-tubulin and anti-fenestrin antibodies in the same way as in undisturbed conjugants. The apical region of the separated conjugants (the post-junction area), corresponding to the junction area of conjugants was labelled with anti-fenestrin antibody and maintained the properties required for the nuclear development. The results of the genetic study were consistent with a hypothesis that cytogamy (pronuclear fusion) was induced in the separated conjugants. Therefore, the lasting cell contact is not necessary for the successful completion of conjugational nuclear events.

A cytogenetic study of development in mechanically disrupted pairs of Tetrahymena thermophila

We examined the nuclear behavior of mating Tetrahymena cells that had been mechanically disrupted at various times throughout conjugation. Disruption was achieved by agitating conjugating Tetrahymena in the presence of 0.1-3 mm glass beads. Two minutes of agitation with 1 mm beads yielded optimal pair disruption (70%) with high viability (92%). Disrupting pairs between 0-4.7 h after the initiation of mating produced mostly disrupted conjugants in which development was aborted. However, as many as 20% of these early disrupted conjugants completed development even without their mating partners. After 5 h the percentage of disrupted conjugants completing development increased dramatically, reaching 80% by 6.7 h. These results support a model suggesting that events associated with nuclear exchange and fusion 5 h into conjugation trigger a commitment to completion of the postzygotic developmental program. The early conjugants that completed development following disruption suggest that development can be sustained even in the absence of a mating partner. This represents a novel method of bringing the micronuclear genome into macronuclear expression with minimal cytoplasmic exchange between partners. We discuss these results in light of a model relating cortical and nuclear signaling events that reciprocally drive conjugal development.

A mutational analysis of conjugation in Tetrahymena thermophila. 2. Phenotypes affecting middle and late development: third prezygotic nuclear division, pronuclear exchange, pronuclear fusion, and postzygotic development

Conjugation following pair formation in Tetrahymena can be divided into three distinct sequences of events: prezygotic development, postzygotic development, and exconjugant development. The decision to proceed with postzygotic development is governed by a developmental checkpoint occurring sometime during the middle stages of conjugation. A second developmental decision is made to initiate pair separation and exconjugant development. This paper examines the phenotypes of five newly isolated conjugation mutants (cnj6-cnj10) which affect middle and late events within the conjugation program. cnj6 mutants exhibit normal nuclear behavior throughout development up to and including differentiation of new macronuclear anlagen. Pairs arrest at this developmental endpoint, unable to dissociate. cnj7 and cnj8 eliminate the third prezygotic nuclear division and the first postzygotic nuclear division. All subsequent developmental events appear normal. cnj9 eliminates the second postzygotic nuclear division, and subsequently, new macronuclei fail to develop despite parental macronuclear degradation. cnj10 results in a pleiotropic phenotype characterized by failure of numerous events which all appear to involve nuclear-cytoskeletal interactions. These defects include nuclear selection (anchoring nuclei to the exchange junction), pronuclear exchange, pronuclear fusion, and anchoring postzygotic nuclear division products to the posterior cell cortex. These mutant phenotypes are used to draw inferences regarding developmental dependencies that govern a cell's entry into the postzygotic and exconjugant developmental programs.

Mutation affecting cell separation and macronuclear resorption during conjugation in Tetrahymena thermophila: early expression of the zygotic genotype

A new recessive conjugation lethal mutation was found in Tetrahymena thermophila which was named mra for macronuclear resorption arrest. Other events affected by the mra mutations are separation of pairs, DNA replication in the macronuclear anlagen, and resorption of one of the two micronuclei. In wild-type crosses 50% of the pairs had separated by 12 hr after mixing two mating types and had completed resorption of the old macronucleus 1-2 hr later. In contrast most mra conjugants did not separate even by 24 hr after mixing and the old relic (condensed) macronucleus was seen in over 90% of them. After addition of 10 mM calcium to the conjugation medium, the mra conjugants did separate but they still failed to complete resorption of the old macronucleus and to replicate macronuclear anlagen DNA in the exconjugants. The calcium induced separation of the mra conjugants occurred later than the separation of control pairs. During normal conjugation cell separation occurs before the first expression of known macronuclear genes and prior to processing of the macronuclear DNA. Therefore, the mra phenotype infers that separation of conjugants requires a signal which is produced by the macronuclear anlagen at an unusually early time.

Conjugal blocks in Tetrahymena pattern mutants and their cytoplasmic rescue. I. Broadened cortical domains (bcd)

The cortical pattern mutant broadened cortical domains (bcd) in Tetrahymena thermophila is unable to complete the nuclear events associated with conjugation. bcd x bcd pairs become arrested at the "nuclear exchange" configuration. Genetic analysis reveals that the bcd conjugal block is 100% penetrant, under macronuclear control, and rescueable (a) by outcrossing to a wild-type partner, (b) by administration of a hyperosmotic shock 5 hr after cells are mixed for mating, or (c) by cytoplasmic transfusion from a wild-type donor. Cytological analysis reveals that the conjugal block is primarily the result of failure in pronuclear fusion (karyogamy). bcd pairs also exhibit reduced nuclear exchange efficiency and a failure of macronuclear anlagen formation. The hypothesis is proposed that the bcd+ gene codes for a microtubule-based organelle "motor" similar to kinesin.